We have had the Superfast Fibre Halo 1 for some months now .. the speed promise says
Post your hub stats. If hh6 then advanced settings then technical log information
enter phone number and post results. https://www.dslchecker.bt.com/adsl/adslchecker.welcome
I am in an almost identical situation to yourself. I had my Superfast Fibre 2 which says an average of 67MBps. Like you and Engineer came out to discover I was getting 35MBps. He said that he would get BT to send another engineer to check it but, instead, I get a text that says I am satisfied with my speed as it's 56MBps.
Done this evening on BT's own speedchecker.
Oh, and their diagnostic tool lies. Let me explain. Runs through a myriad of checks and tells you to unplug the faceplate (which I did) and connect the hub directly into the master socket inside, which I did.
It then says that everything is fine and I should pay for and engineer at £129 as it must be my equipment.
So, I run the test again, this time leaving my router connected into the master socket. It then shows my speed below and tells me to connect to the master socket (which it already is). THEN it tells me that the Master Socket is fine and it has to be my equipment. So, to my mind, it's twoddle. Their Diagnostic tool is rubbish.